The present invention relates to a method and apparatus for maintaining a desired gas-liquid absorption level in a liquid irrespective of changes of temperature of the liquid. More specifically, the present invention relates to a method and apparatus for controlling the pressure of CO.sub.2 gas supplied to a carbonator in order to maintain a desired carbonation level of the carbonated liquid therein, despite changes in temperature of the carbonated liquid.
Carbonators produce carbonated water of predetermined (target) carbonation levels, by absorbing CO.sub.2 in water. The driving force for mass-transfer in the absorption process is equal to the difference between the saturated concentration of CO.sub.2 in water and the target carbonation level. Since other mass-transfer conditions within the carbonator, such as contact surface and agitation are normally fixed, the target carbonation will be maintained provided that this driving force for mass-transfer is kept constant. However, the saturated concentration of CO.sub.2 in water is dependent on pressure and temperature, according to Henry's law. As a consequence, when the water temperature varies, the resulting change in mass-transfer driving force will cause a change in the level of final carbonation and the target carbonation level will thus not be maintained. For example, when the temperature of water in the carbonator increases, it is necessary to increase the pressure of carbon dioxide supplied to the carbonator in order to maintain a given carbonation level. For this reason, carbonators are usually designed to operate within a very narrow water temperature range and the CO.sub.2 pressure is fixed to that the target carbonation level is attained within this temperature range.
In order to achieve the target carbonation level, regardless of the prevailing water temperature in the carbonator, the CO.sub.2 gas pressure must be varied with water temperature, ideally so as to produce a constant driving force (i.e. a constant difference between saturated concentration and target concentration). Simply changing gas pressure in relation to water temperature is not reliable, since this does not have a linear relationship to mass-transfer driving force, as illustrated by FIG. 1. No simple, direct-acting device exists either for controlling carbonator gas pressure according to water temperature, or more particularly, for controlling mass-transfer driving force according to water temperature.